Converting Time Units Without Tripping Over the Calendar
Up to a week, time is simple multiplication. Past a week it stops being arithmetic, because months have different lengths and years occasionally have an extra day. The Time Converter handles the part that is reliable.
The clean conversions
Sixty seconds to a minute, sixty minutes to an hour, twenty-four hours to a day, seven days to a week. Those four never vary.
Worth memorising: an hour is 3,600 seconds and a day is 86,400. The second of those turns up constantly in anything technical, because it is the number a Unix timestamp advances by each day.
A week is 604,800 seconds, which is the last figure you can state without an argument.
Where it breaks
A month is not a unit. It is 28, 29, 30 or 31 days, so "three months" is somewhere between 89 and 92 days. Software that treats a month as 30 days drifts by five days a year, which is how a subscription renews on a date nobody chose.
A year is 365 days except when it is 366. Leap years are every four years, except centuries, except centuries divisible by 400 - so 1900 was not a leap year and 2000 was. The average works out at 365.2425 days, which is the figure to use for long spans and never for a specific one.
So the rule is: for anything involving a real date, use dates rather than multiplication. Convert both ends and subtract, which is what the Date Difference Calculator does.
The things that break the clock
A day is not always 24 hours of local time. Where clocks change, one day a year is 23 hours and another is 25, and code that adds 86,400 seconds to get "tomorrow at the same time" is wrong twice a year.
Leap seconds are the other one - occasionally added to keep clocks aligned with the Earth's slightly irregular rotation. Unix time ignores them entirely, pretending every day has exactly 86,400 seconds, which is a deliberate simplification rather than an oversight. The Unix Timestamp Converter works in that simplified world, as almost all software does.
Common questions
How many seconds in a year?
About 31.5 million - 31,536,000 for a 365-day year. The approximation "pi times ten million" is accurate to half a per cent and oddly memorable.
How many weeks in a month?
Between four and four and a third. There is no clean answer, which is why payroll runs on months or fortnights and never on "four weeks" unless it means exactly that.
Why does a year have 52 weeks and a day left over?
Because 52 times 7 is 364. That spare day is why a given date moves forward one weekday each year, and two after a leap year.
What is a fortnight?
Fourteen days. Still in everyday use in Britain and Australia, and almost nowhere else.
Got a duration to convert?